This invention relates to an exhaust gas recirculation (referred hereinafter to as "EGR") control system for a diesel engine, and more particularly to an improvement in an EGR control system to optimize both NOx (nitrogen oxides) emission control and engine driveability.
Many modern internal combustion engines are equipped with EGR control systems in which a part of an engines exhaust gas is recirculated back to the engine to suppress a rise in combustion temperature in the combustion chambers of the engine so as to lower NOx emission from the engine. In such engines, it is required to control quantities EGR gas to optimize both NOx emission decreasing effect and engine driveability. The optimal quantity of EGR gas usually depends on the pressure differential between intake air and exhaust gas and on the opening area of an EGR passageway connecting intake and exhaust passageways.
In diesel engines, a throttle valve is provided to generate intake vacuum necessary for EGR, in which the throttle valve is operated in relation to a fuel control lever of a fuel injection pump which control lever is moved by an accelerator. However, with this arrangement, an increased accelerator operation effort is unavoidably required; and since the displacement of the fuel control lever or fuel injection amount is greater at a low engine speed and high engine load operation range, EGR further deteriorates combustion in the engine, thereby increasing emission of black smoke. Otherwise, there is a device for hydraulically controlling the throttle valve without connection with the accelerator. Even with this device, however, the problem of increased black smoke emission has not been solved.